1. Field of the Invention
The invention relates to a method of production of secondary steel based on scrap, wherein the scrap is fed in a scrap preheater through a charging device, is heated there and, finally, is brought in a smelting unit and is melted, wherein process gas which leaves the smelting unit, is used for preheating of the scrap, is freed of harmful materials and dust in a reheating device and in an adjoining dedusting installation, respectively, and is discharged from the installation as a cooled cleaned flue gas.
2. Description of the Prior Art
With such a method, the scrap is fed through a charging device in a preheater which, as a rule, is heated directly with a hot flue gas from a smelting unit, e.g., electrical arc furnace. It is mostly aspirated through the preheater, heating the scrap. The flue gas is cooled off. The preheated scrap (dependent on the process, to about 600-800° C.) is then transferred in a smelting unit and is melted with electrical energy. In addition, a fossil energy carrier (natural gas, coal, oil) is also used. As oxidation means, as a rule, technical oxygen (>95% by volume of O2) is used. As additives for forming the flux, mostly mineral products (lime, dolomite) are used.
Dependent on the process conditions, the parameters (temperature, composition, oxygen content, dust content, amount) of the flue gas (process gas), which is produced during the melting process, are subjected to strong variations. In addition, the melting process is not continuous but rather discrete (batch operation).
The scrap contains, dependent on quality, type, and provenance, other accompanying products. The composition and amount of these mostly hydrocarbon-containing accompanying products (oils, fatty products, cooling and lubrication materials, coatings, etc.) differs to a very large degree.
The changeable flue gas values result, therefore, in strong variations of the operational parameters of the preheaters. This leads to undefined and non-stationary oxidation conditions of accompanying products which cling to the scrap. This is favorable to the formation of undesired flue gas components (CO, non-combusted hydrocarbons, aromates, chloride, . . . ). Those are produced mostly during evaporation and/or (partial) oxidation of the components clinging to the scrap.
In order to meet the legal requirements, an expensive flue gas treatment is necessary to eliminate and separate the non-combusted hydrocarbons and dioxins/furans. To this end, the flue gas is heated, after being discharged from the preheater, in a reheating chamber to a T>850° C. and after a certain dwell time of t>2c is rapidly cooled to about 200° C. (is quenched). Thereby the new formation (denovo-syntheses) of dioxins/furans (PCDD/F) should be suppressed. In a following process stage, separation of the residual PCDD/F takes place by injection of, e.g., lignite-coke dust (entrained-phase adsorption). The injected coke dust then is separated, together with the conventional fine dust, in a dedusting installation.
This type of a process insures maintaining of emission threshold for PCDD/F of 0.01 ng TE/Nm3 but, however, is associated with increased additional expenses (for the primary energy for the burner, cooling water for quenching, waste water treatment system for quenching, silo, metering and delivery technique for the coke dust, etc.). In addition, the coal content in the filtered dust is increased above the permitted limit so that a thermal secondary treatment of the filtered dust before depositing is necessary.
The sum of these expenses exceeds the advantages (saving of electrical energy, increase of the productivity) that could have been expected from the use of the scrap preheating system. In addition, additional devices become responsible for additional disturbances in the installation.
In order to minimize these drawbacks, WO 03/068995 A1 suggests a smelting installation for a continuous production of steel with the use of metallic materials. The used materials such as, e.g., scrap, steel sponge or the like, are preheated in an upper part of a smelting vessel, which is formed as a shaft, and then are melted in its lower portion of the vessel with fossil fuels. The produced melted product is continuously fed in an adjacently arranged treatment vessel which is formed as an electrical arc furnace, and is brought there to the desired steel quality with electrical energy. For post-combustion, the post-combustion gases are delivered in different planes in the material column from outside and from inside through an inner shaft extending in the middle of the material column and with which a staged post-combustion of the ascending process gases is achieved, together with the reduction of oxidation of the iron-containing initial material.
With this known process, only the primary energy is used for melting the scrap. In order to prevent oxidation of iron, heating in the melting stage should be carried out non-stoichiometrically with an air ratio in a range 0.5-0.9. Therefore, the flue gas contains, after leaving a separate melting stage, still a large amount of combustible components (in particular CO, H2 and CH4). This results in a poor use of the used energy carrier. Therefore, further measures are necessary in order to improve the energy use which results in increased costs. To this end, there is provided means for feeding the reheating air in the shaft above the melting zone (integrated after burning). This measure improves the energy use, however, for different reasons, probably, no complete conversion of non-combusted materials in the shaft is possible. Moreover, the flue gas should be withdrawn at the shaft outlet with a temperature about 800° C. in order to prevent an additional post-combustion stage.
It is an object of the invention to provide a method with which the described drawbacks of the preheating and melting with an exclusive use of primary energy, are reliably prevented or at least are minimized.